An organic electroluminescence device (hereinafter, “electroluminescence” may be abbreviated as “EL”) is a spontaneous light emitting device which utilizes the principle that a fluorescent substance or a phosphorescent substance emits light by energy of recombination of holes injected from an anode and electrons injected from a cathode when an electric field is applied. Since an organic EL device of the laminate type driven under a low electric voltage was reported, many studies have been conducted on organic EL devices using organic materials as the constituent materials. The devices of the laminate type use tris(8-quinolinolato) aluminum for a light emitting layer and a triphenyldiamine derivative for a hole transporting layer. Advantages of the laminate structure are that the efficiency of hole injection into the light emitting layer can be increased, that the efficiency of forming exciton which are formed by blocking and recombining electrons injected from the cathode can be increased, and that exciton formed within the light emitting layer can be enclosed. As described above, for the structure of the organic EL device, a two-layered structure having a hole transporting (injecting) layer and an electron transporting light emitting layer and a three-layered structure having a hole transporting (injecting) layer, a light emitting layer, and an electron transporting (injecting) layer are well known. To increase the efficiency of recombination of injected holes and electrons in the devices of the laminate type, the structure of the device and the process for forming the device have been studied.
As the light emitting material of the organic EL device, a metal complex such as a tris(8-quinolinolato) aluminum complex, a coumarine derivative, a tetraphenylbutadiene derivative, a distyrylarylene derivative, and an oxadiazole derivative are known. It is reported that light emission ranging from blue light to red light in the visible light region can be obtained by using those light emitting materials, and a device exhibiting color images was realized.
A fluorescent light emitting material that emits light by means of a singlet exciton has been conventionally used as a light emitting material for an organic EL device. In recent years, the utilization of a phosphorescent light emitting material that emits light by means of a triplet exciton as well as the fluorescent light emitting material has also been proposed (for example, Non Patent Literature 1 and Non Patent Literature 2). An organic EL device using the phosphorescent light emitting material can achieve luminous efficiency three to four times as high as that of an organic EL device using only the fluorescent light emitting material because it is assumed that singlet excitons and triplet excitons are produced at a ratio of 1:3 upon recombination of electrons and holes in an organic EL device by virtue of a difference in spin multiplicity. In blue phosphorescent light emission, however, high efficiency and a long lifetime are hard to achieve, and hence the development of a host material that achieves the high efficiency and the long lifetime has been desired.
Patent Literature 1 proposes a compound in which two carbazole skeletons are bonded to each other through a linking group. Patent Literature 2 describes a compound in which two carbazole skeletons are bonded to one dibenzothiophene skeleton (for example, Compound 24). Patent Literature 3 describes a compound in which two carbazole skeletons and two dibenzothiophene skeletons are combined with one another (for example, Compound 21).
However, none of those literatures describes a compound having two carbazole skeletons bonded to each other through a linking group as required in which a dibenzothiophene skeleton or a carbazole skeleton is bonded to at least one of the carbazole skeletons at its N-position through a linking group as required.
Patent Literature 4 describes a compound (H-12) in which two carbazole skeletons are bonded to each other through an m-phenylene linking group and each of the carbazole skeletons is substituted at its N-position with a dibenzothiophene skeleton through an m-phenylene linking group.
However, the compound has a large molecular weight and is considered to have a large intermolecular force because its molecular skeleton is linear (one-dimensional). Accordingly, it is assumed that the compound has low sublimation property and hence requires a high temperature in its sublimation step or vapor deposition step, with the result that the step involves its thermal decomposition. Therefore, the compound is considered to be unsuitable as a material to be used in an organic EL device produced by employing a sublimation step or a vapor deposition step.
In addition, Patent Literature 4 intends to provide, for example, an organic EL device showing a small number of dark spots, a small leak current, and small light emission unevenness in the light emitting device. The literature does not reveal whether the organic EL device emits phosphorescence with high efficiency at a low driving voltage.
Further, Non Patent Literature 3 describes a compound (Compound 8) having two carbazole skeletons in which each of the carbazole skeletons is further substituted at its N-position with a carbazole skeleton at its 3-position. However, the compound has a low glass transition temperature because the compound has a long-chain alkyl group. In addition, it is assumed that owing to its molecular weight and an effect of the entanglement of its long-chain alkyl groups, the compound has low sublimation property and hence requires a high temperature in its sublimation step or vapor deposition step, with the result that the step involves its thermal decomposition. Therefore, the compound is considered to be unsuitable as a material to be used in an organic EL device produced by employing a sublimation step or a vapor deposition step.
In addition, a blue phosphorescent light emitting device using any one of the compounds described in Patent Literatures 1 to 3 has had an insufficiently low driving voltage, insufficiently high efficiency, and an insufficiently long lifetime.